In many WLAN and other wireless data networks implementations, it is beneficial for the system owner to know the physical location of mobile clients or compatible tags. This will enable new features such as enhanced network security, providing of ‘location based’ services, asset tracking and many others
The patent applications mentioned above, describe the basic techniques for performing ‘location’ of a standard client or ‘stand alone’ tag. A typical ‘location finding’ system, as currently implemented by the assignee and disclosed in the related patent applications, consists of multiple ‘location receivers’ connected to the WLAN system, either by means of CAT-5 backbone or by wireless bridges. The typical ‘location receiver’ contains a WLAN receiver and the circuitry required to extract Time of Arrival (TOA) information and report this information to the system's location server. The ‘location server’ performs the required computation of the client location based on the known location of the location receivers, and displays it to the user or reports it to the requesting application.
In a TDOA based location system, the TDOA of each pair of location receivers is calculated from the reported TOA's that were calculated on a single transmitted message. For this reason, all those location receivers reporting a single transmitted message are all tuned to the same channel.
In a wireless local area data communication system, said Location Receivers may be attached and/or integrated and/or be a part of the Access Points in said wireless network. Further elaboration of these systems and methods can be found in the above mentioned patent application.
In some location systems it is desirable to have the location receivers or Access Points (AP) able to measure TOA of a received message, working in different channels. In such a system, a tag or standard client is required to transmit several messages at different channels to allow all those location receivers or AP's to receive and measure the TOA of the transmitted messages. Since the transmission of those consecutive messages is not synchronized, it's not possible to calculate the TDOA from the reported TOA of two receivers at different channels. In those cases, TDOA is calculated between receivers tuned to the same channel (reporting the TOA of a single message).
Normally the transmission of those messages by a standard client or a tag depends on clear channel assessment (CCA) and for that reason accurate transmission time cannot be ensured. Moreover, the transmission of messages is initiated by a processor which cannot ensure accurate start of transmission.
To perform an accurate TDOA location, it's needed to have a timing uncertainty between transmitted messages of no more than few nsec. Current transmitter units cannot achieve such accurate transmission timing and still be compliant with other network transmission requirements.
Therefore, a need existed to provide a device and method that overcomes the above problems.